Uranium alloys



URANIUM ALLOYS Alan U. Seyholt, Scotia, N. Y., assignor to the United States of America as represented by the United States Atomic Energy Commission No Drawing. Application June 7, 1948 Serial No. 31,599

20 Claims. (Cl. 75-122 .7)

This invention'relates to metal alloys and more particularly to certain alloys containing a major weight proportion of uranium.

Substantially pure uranium is desirable for use in neutronic reactors and neutron multiplying systems, but it readily corrodes, is dii'iicult to work and is diificult to fabricate because of its rather low mechanical strength. Mixtures or alloys containing a major proportion by weight of uranium and a minor proportion by weight of other elements may be used in neutronic reactors such as that described in the Fermi-Szilard U. S. Patent No. 2,708,656, dated May 17, 1955, filed December 19, 1944, as application Serial Number 568,904, as long as the modifying element does not cause a material increase in the neutron capture cross section for the uranium alloy. When elements other than the light elements are added to uranium the slow neutron capture cross section of the product has been found to be intermediate between thecapturecross section of the uranium and the alloying element, in proportion to the percentages of each. Furthermore, light elements often cause undesirable modification of the energy of neutrons in a neutronic reactor. In some situations, they undergo undesirable nuclear re actions. Consequently, there is a severe limitation on the materials which may be used as alloying elements.

Alloys containing a major proportion'of uranium have been manufactured before the time of this invention, such as uranium-zinc, uranium-manganese, uraniumcadmium-and like alloys, but in all these the alloying element has a rather high neutron capture cross section and a number of these alloys are also very easily corroded. In the past a few compositions have been made in which a minor proportion by weight of uranium was alloyed with a major proportion by weight of elements having a small neutron capture cross section. For example, U. S. Patent 1,759,454 granted to John Allen Heany teaches a composition of uranium and molybdenum. This composition, however, contains a maxi- 'mum of percent uranium and is not generally satisfactory for neutronic reactors.

The primary object of this invention is therefore to provide a composition which contains a major proportion of uranium and which has a capture cross section for thermal neutrons not materially greater than that of uranium metal. 7

Another object of this invention is to provide a composition which contains a major proportion of uranium and which is corrosion resistant.

Another object of this invention is to provide a composition which contains a major proportion of uranium and which has desirable tensile and other physical proper ties.

A further object of this invention is to provide a uranium composition which is corrosion resistant, which has high tensile and compressive strength, which is of increased hardness that may be modified by heat treatment, which is strong, ductile, and easily fabricated, and

States tent which has a capture cross section for neutrons not substantially greater than that of uranium metal.

Still further objects and advantages will become apparent to those skilled in the art. from the description and examples which follow;

The objects of the invention are achieved by preparing an alloy contining a major proportion by weight of uranium and a minor proportion by weight of at least one alloying metal of the fourth period of the class consisting of zirconium, columbium, and molybdenum.

More particularly uranium alloys containing from 0.1 to 10 percent'by weight, but preferably at least 5 percent, of these alloying elements exhibit highly desirable nuclear and structural properties.

The method of the invention can best be illustrated by examples showing the preparation of these alloys. Although the alloys can be prepared on various scales, the following examples illustrate the preparation of relatively small samples. These examples are given for the purpose of illustration only and are not for the purpose of limiting the spirit or scope of the invention.

EXAMPLE I 262.9 grams of uranium and 13.8 gramsof zirconium hydride (ZrH are placed in a beryllium oxide crucible. The beryllium oxide crucible is placed within a graphite crucible and this in turn is placed in a fused silica tube, which is connected in series to a condenser and to a pump capable of evacuating the system to 0.1 micron of mercury absolute pressure. The fused silica tube is placed within an induction furnace. The system is evacuated to about 1 micron of mercury. The furnace is turned on and the temperature increased to 1350 C. The zirconium hydride dissociates as it is heated and the hydrogen given off increases the absolute pressure to about 300 microns. As soon as the hydrogen is all removed the absolute pressure is reduced again to 1 micron. The

The sample has improved tensile properties as cast and these may be further improved by proper heat treatment. This is shown by the results of hardness tests. The sample as cast has a Rockwell hardness of RA, as compared with a hardness of pure uranium as cast of 50 RA. This hardness can be increased to a 63 RA hardness by heating the sample to 900 C. for two hours in a vacuum furnace, and quenching it in Water. When the sample in reheated to 300 C. for 2 hours and again quenched in water, the hardness is further increased to RA 64.

"the uranium-zirconium alloy has greater corrosion resistance than substantially pure uranium.

- ticularly true when the uranium-zirconium alloy is heat treated. Substantially pure uranium as cast loses 2.7 mg./cm. /hr. when exposed to boiling distilled water. The corrosion loss of pure uranium can be reduced by appropriate heat treatment. For example, when sustantially pure uranium is heated to 900 C. for 4 days, and

a after an initial weight loss of less than 50 mgJcmF.

This is par-' When alloys are prepared according to the method of Example I, segregation may occur if mixing is incomplete. To avoid this, melts may be made by use or" a bottom pouring arrangement. The following example illustrates the preparation of a weight percent uraniumcolumbium alloy using such bottom pouring.

EXAMPLE II 212 grams of uranium and 11.2 grams of columbium are carefully weighed out into a beryllium oxide crucible, which contains a pouring hole in the center of its bottom closed by a close-fitting graphite pouring rod. The beryllium oxide crucible is placed within a graphite crucible, which also has a small hole in its bottom in line with the pouring hole in the beryllium oxide crucible. The assembled graphite crucible is placed above a graphite mold in a'fused silica tube which has connections attached to a vacuum system. The upper end of the pouring rod is passed through the fused silica tube by means of a flexible air-tight seal. The system is pumped down to 5 microns of mercury absolute pressure. Argon is then admitted until the system is at a positive pressure of about 20 millimeters of mercury, Heat is applied by means of an induction furnace. When the temperature reaches l350 C., it is maintained at this value for minutes, causing the uranium and columbium to melt and tomix thoroughly. The induction furnace is then turned off. The pouring rod is removed from the hole and the molten alloy drops into the graphite mold, where it forms a well-consolidated ingot about 1 inch in diameter and 1V: inches long.

The alloy as cast has improved mechanical properties over those of pure uranium and these properties may be further improved by appropriate heat treatment. This is illustrated by the results of Rockwell hardness tests on a five percent columbium-95 percent uranium alloy as cast, after heating to 900 C. for 2 hours in a tank hydrogen atmosphere and quenching in cold water; and after successive beatings wrapped in copper foil at temperatures between 300 C. and 700 C. followed by quenching in cold water. The results of these tests plus those of testing pure uranium given for the purpose of comparison are as follows:

It may be seen that the 5 percent c'olumbium alloy as cast and especially after heat treatment shows even greater improvement over that of pure uranium than the 5 percent zirconium-uranium 'alloy. Thereis an optimum heat treatment for maximum hardness which in the case of the 5 percent columbium alloy for the various heat treatments tried is heating to 900 C. for 2 hours, water quenching and heating to 500 C. for 2 hours. There is an improvement in tensile strength corresponding to this increase in hardness in the alloys.

Example III, which follows, illustrates a presently preferred embodiment of the invention.

EXAMPLE III crucible, which. also has:a smallhole "in'line'with the" pouring hole in the beryllium oxide crucible. The assembled graphite crucible is placed above a graphite mold in a fused silica tube which has connections attached to a vacuum system. The pouring rod is passed through the fused silica tube by means of a flexible air-tight seal. The system is evacuated to 10 microns of mercury absolute pressure and is maintained at this absolute pressure. Heat is applied by means of an induction furnace. After the temperature reaches 1390 C., it is maintained at this value for 10 minutes. The pouring rod is then elevated through a Wilson seal, allowing the metal to run into a /2 inch x 2 inch cylindrical graphite mold which fits into a groove on a sooted copper stool to insure freezing from the bottom. This method yields sound, uniform cylindrical castings suitable for machining to ,4; inch x 1 inch cylinders.

The 5 percent molybdenum-uranium alloy has greatly 7 improved mechanical properties as compared with the properties of uranium metal. Its hardness as cast is 69 RA which is greater than either the 60 RA of the as cast 5 percent uranium-zirconium alloy or the 67 RA of the as cast 5 percent uranium-columbium alloy. When heated to 900 C. and quenched, the hardness decreases toRA 66, but when the sample is reheated to 400 C. for twohours and quenched in water, the hardness increases to 76 RA.

This 5 percent molybdenum alloy also shows greatly improved mechanical properties. In order to evaluate these properties, two samples of substantially pure uranium were tested in compression at the same time as a 5 percent molybdenum-uranium alloy which had been annealed 2 hours at 900 C., quenched and reheated 2 hours at 300 C. and quenched in water. The uranium sample has a 0.2 percent offset yield strength of 28,000 p. s. i, it flattens without fracture having a 50 percent reduction at 200,000 p. s. i, and has a Rockwell hardness of 50 RA. The quenched and tempered 5 percent molybdenum sample has a 0.2 percent offset yield strength of 206,000 p. s. i., a fracture stress on compression of 287,00 p. s. i, and a hardness of 76 RA. It may be seen that the uranium-molybdenum alloy has almost 10 times the 0.2

percent offset yield strength of pure uranium.

It has been pointed out in Example I that change in the heat treatment changes the rate of corrosion of uranium-zirconium alloys. This is also true of uraniummolybdenum alloys and is illustrated by results of tests in boiling distilled water on two 5 percent molybdenum alloys with different heat treatment. One alloy sample was heated to 850 C. for 2 hours and water quenched; and another alloy sample was heated the same amount, quenched, and then annealed by heating to 350 C. for 24 hours and then water quenched. The annealed alloy is more corrosion resistant than the alloy which is only heated and water quenched. When tested in boiling distilled water, at the end of an eighty-day period, the water quenched alloy corrodes ten times as fast as the annealed alloy. After 16 days at C., the rate of corrosion of the water quenched alloy is 0.2 mg./cm. /hr., whereas the rate of corrosion for the annealed alloy is 0.03 mg./ cmF/hr. Pure uranium tested under the same conditions has a corrosion rate of 3 mg./cm. /hr.

When tested in an autoclave at elevated steam temperatures, the annealed alloy is again superior to the pure uranium and non-annealed alloy.

As stated before, the improved hardness of the alloys is a good indication of the improved mechanical properties such as tensile strength. A 2 percent molybdenum-uranium alloy which is heated to 700 C. for 24-hours and water quenched has a 0.2 percent offset yield of 91,500 p. s. i. and a modulus of elasticity of 16 10 .The stress required for plastic deformation rises quite rapidly with increased strain until at 30 percent reduction in height, a stress of about 300,000 p. s. i. is required. Pure uranium as cast compresses 50 percent at 200,000 p. s. i. When the quenched2 percent alloy is annealed at 300 C. or

'- 400 C. for a short period of time and quenched, the

annealed specimens are characterizedby a much steeper rise of stress with strain than in the case of the as quenched material. For example, when the 2 percent quenched alloy is annealed for 2 hours at 400 C. and water quenched, the offset yield is increasedto 237,000 p. s. i. and the modulus of elasticity to 23.5 X When the quenched 2 percent sample is annealed for 2 hours at 300 C., a stress of 300,000 p. s. i. is required for a 2 percent reduction in height. Annealing one hour at 300 C. does not appear to cause as much resistance to deformation as two hours at this temperature. However, nearly 50 hours at 400 C, is not very effective in cansing high resistance to flow, and the very long aging treatments at this temperature do not cause excessive brittleness.

Alloys such as those illustrated in the prior examples, containing 5 percent of zirconium, columbium, or molybdenum or other alloys containing from 1 to 10 percent of any of these elements have desirable nuclear properties. he neutron capture cross sections at thermal energies for zirconium, columbium,.and molybdenum are all below that of natural uranium which is about 3 barns. Uranium alloys containing from 1 to 10 percent of any of these elements also have low neutron capture cross sections. Any

of these alloys may thus be conveniently used in a chain reacting pile wthout requiring an appreciably greater amount of these alloys than pure uranium.

It has been pointed out hereinbefore that change in heat treatment changes both the compression and tensile properties and the corrosion resistance properties of 5 percent alloys of zirconium or columbium with uranium. Change of composition also changes other properties of the alloys. The effect of change in composition is illustrated by the results of hardness tests of a number of the as cast uranium molybdenum samples containing varying amounts of the alloying elements. The results are given in Table 1 below:

Table 1 Weight percent Molybdenum.-. .7 2.0 2.6 3.9 5 10 Hardnessas cast 64 68 67 69 69 58 Heat treatment-heated to 900 7 0., quenched, heated to 300 C. for 2 hours, and quenched 72 76 76 78 76 62 Changing the composition will also change the as cast rate of corrosion. This is illustrated by the results of corrosion tests on a number of as cast molybdenum alloys tested in boiling distilled water which are as follows:

Table 2 As Cast, Percent Molybdenum Hours in Test Boiling Distilled H2O Corrosion Rate, mgJcmJ/hr.

less than 5. less than 3. less than 2. less than 1. less than 1. less than 0.2.

The temperature to which the as cast samples are heated in order to improve their properties may be varied. For most concentrations 900 C. is preferable. The annealing temperature may also be varied. The range 300- 400 C. is the most desirable. An annealing temperature of 300 C. usually causes the greater improvement in mechanical strength and tensile properties.

This is fortunate, because uranium oxidizes much more rapidly at 400 C. than at 300 C., hence a one-hour low temperature treatment at 300 C. appears to be most practical from the standpoint of securing nearly maximum strength.

The foregoing examples have pointed out that alloys of uranium can be prepared with zirconium, columbium, and

alloying element and/or because of the heat treatment.

- The changes may also be caused by the formation of intermetallic compounds, by precipitation hardness, by changes in crystal sizes, or by a combination of such occurrences. 4

Advantage may be taken of the change in properties in many ways. For example, the density ingrams per cubic centimeter of uranium in the alpha or beta phase is- 19.0, and in the gamma phase is 18.6. Under the proper circumstances the addition of zirconium, columbium, or molybdenum causes the uranium-to remain in the gamma phase, and under other conditions the addition of molybdenum causes the uranium to remain in the beta phase. It is therefore possible to obtain an alloy containing at least percent uranium with a desired density and other physical properties by adding the proper amount of the alloying element and giving a proper heat treatment.

Many other modifications of the method of this inven tion are also possible. For example, the alloys can be made on as large a scale as is desirable. Amaster alloy can be prepared, containing a large amount of the alloying element. This ailoy is then ground up, carefully analyzed, and alloys of any desired composition may be made by merely adding additional uranium.

Because there are many modifications which may be made, it is to be understood that this inventionis not to be limited except as indicated in the appended claims.

What is claimed is:

1. A binary alloy of uranium and from 0.1 to 10 percent by weight of a metal of the fourth period of the class consisting of zirconium, columbium, and molybdenum.

2. An annealed binary alloy of uranium and from 0.1 to 10 Weight percent of one metal of the fourth period of the class consisting of zirconium, columbium, and molybdenum.

3. An annealed binary alloy of uranium and from 0.1 to 10 percent by weight of zirconium.

4. A binary alloy of uranium and from 0.1 to 10 weight percent of zirconium.

5. A binary alloy consisting of about percentby Weight uranium and about 5 percent by weight zirconium.

6. An. annealed binary alloy of uranium and from 0.1 to 10 percent by weight of columbium.

7. A binary alloy of uranium and from 0.1 to 10 weight percent columbium.

8. A binary alloy consisting of about 95 percent by weight of uranium and about 5 percent by weight of columbium.

9. An annealed binary alloy of uranium and from 0.1 to 10 percent by weight of molybdenum.

10. A binary alloy of uranium and from 0.1 to 10 weight percent molybdenum.

11. A binary alloy consisting of about 95 percent by weight of uranium and about 5 percent by weight of molybdenum.

12. The method of improving the properties of binary alloys of uranium consisting essentially of uranium and from 0.1 to 10 percent by weight of a metal of the fourth period of the class consisting of zirconium, columbium,

' and molybdenum, which comprises heating the uranium alloy to about 900 C. for an extended period and rapidly quenching it.

13. The method of improving the properties of binary uranium alloys of uranium and from 0.1 to 10 percent uranium alloys consisting essentially of uranium and from 0.1 to 10 percent by weight of columbium, which comprises heating the uranium alloy at about 900 C. for 24-hours and quenching it in water.

15. The method of improving the properties of binary uranium alloys consisting essentially of uranium and from 0.1 to 10 percent by weight of molybdenum which coinprises heating the uranium alloy to about 900 C. for 24 hours and quenching it in water.

16. The method of improving the properties of binary uranium alloys consisting essentially of uranium and from 0.1 to 10 percent by weight of a metal of the fourth period of the class consisting of zirconium, colurnbiurn, and molybdenum which comprises heating the uranium alloy to about 900 C. for an extended period, rapidly quenching it, reheating it at from 300 to 400 C. for an extended period and rapidly quenching it.

17. The method of improving the properties of binary uranium alloys consisting essentially of uranium and of from 0.1 to 10 per cent by weight of zirconium which comprises heating an alloy to about 900 C. for an extended period, quenching it in Water, reheating it at from 300 to 400 C. for an extended period, and quenching it in water.

18. The method of improving the properties of binary uranium alloys consisting essentially of uranium and from 0.1 to 10 percent by weight of columbium which comprises heating an alloy at about 900 C. for an extended period, quenching it in water, reheating it at from 300 to 400 C. for about 2 hours, and rapidly quenching it in water.

19. The method of improving the properties of binary References Cited in the file of this patent UNITED STATES PATENTS 1,106,384 Hughes Aug. 11, 1914 1,845,145 Grenagle Feb. 16, 1932 OTHER REFERENCES Ahmann et al.: CT-2946 (July 1955); declassified December 15, 1955; 26 pages; available at O. T. S. at $4.80. 

1. A BINARY ALLOY OF URANIUM AND FROM 0.1 TO 10 PERCENT BY WEIGHT OF A METAL OF THE FOURTH PERIOD OF THE CLASS CONSISTING OF ZIRCONIUM, COLUMBIUM, AND MOLYBDENUM. 